If you only ever download one mod for Oblivion, let it be this one. This mod fixes a HUGE number of bugs which Bethesda never fixed.
The full list of fixes can be found here. They report that they have fixed over 1800 bugs.

After v3.2.0, Quarn and Kivan abandoned the maintenance of the UOP, probably to focus on the Unofficial Fallout 3 patch. Fortunately, Arthmoor decided to continue developing the UOP. The link to his verson is above.